1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile apparatus which performs image formation by an electrophotographic method.
2. Description of the Related Art
The image density of an image forming apparatus using an electrophotographic method varies depending on the temperature and humidity condition of the ambient circumstances or the frequency in use of the process station. Hence, the image forming apparatus corrects the variation by controlling the image density. The image forming apparatus forms density patches of the respective colors on a photosensitive drum, an intermediate transfer member (to be referred to as an “ITB” hereinafter), or an electrostatic adsorptive transfer belt (to be referred to as an “ETB” hereinafter). The density patches are read by a density detection sensor and fed back to the process forming conditions. This allows the maximum density and the halftone characteristic of each color to be maintained in the ideal state.
In general, the density detection sensor causes a light source to illuminate a density patch and a light-receiving sensor to detect the reflected light intensity. The signal of the reflected light intensity is A/D-converted, processed by the CPU, and fed back to the process forming conditions.
The methods of density detection sensors are roughly classified into a method of detecting the irregularly reflected components of reflected light and a method of detecting the regularly reflected components of reflected light. The irregularly reflected light detection method is suitable to detect a chromatic color toner but unsuitable to detect a black toner because it detects a reflected component perceivable as a color. On the other hand, the regularly reflected light detection method is more advantageous than the irregularly reflected light detection method because it mainly detects reflected light from the background, and density detection can be done independently of the color of toner/background.
In the density sensor using the regularly reflected light detection method of mainly detecting reflected light from the background, if the surface state of the background varies depending on the frequency in use of the background, the reflected light amount varies, too. Japanese Patent Laid-Open No. 2007-292855 describes that normalizing the reflected light amount of a density patch by the reflected light amount of background (to be referred to as “background correction” hereinafter) is effective. Measurement of the background reflected light amount for background correction is performed at the same timing as density patch creation and at the same position of the background as much as possible in consideration of the material unevenness and time-rate change of the ETB or ITB.
The amount of light emitted by the light-emitting element of the density sensor varies due to the influence of heat generation of the light-emitting element itself and the like. The light emission amount largely varies immediately after the start of energization and then moderately converges along with the lapse of time.
Hence, when the sensor performs detection before convergence of the light emission amount, the detection result contains errors. Read by the density sensor may be started after the light emission amount of the light-emitting element has stabilized. In this method, however, the time required for density measurement is long.